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This regional study of methylmercury in effluent and receiving waters of Central Valley dischargers was initiated by Central Valley Clean Water Association (CVCWA) in response to an order issued by California's Central Valley Regional Water Quality Control Board (Water Board). The order requires Central Valley NPDES permit holders to monitor for methylmercury in their effluent discharges. According to the order, the Water Board believes the monitoring information is necessary to determine the extent to which NPDES permitted facilities are contributing to an impairment of beneficial uses caused by methylmercury.

Methylmercury is the form of mercury known to accumulate to unhealthy levels in fish, threatening the health of people and wildlife that depend on fishable waters for their livelihoods. California, like many areas of the country, has too much mercury in fish. Because methylmercury is seen as impairing the beneficial use of fishing, it has become the subject of a federally required Total Maximum Daily Load (TMDL). The TMDL, to be promulgated by the Water Board, includes the establishment and implementation of numeric targets that indicate fish are safe for people and wildlife to eat. The science of mercury cycling leads the process to consider methylmercury targets, but implementing those targets in a meaningful way can be challenging.

In anticipation of the many technical and scientific issues that will arise in the course of the Water Board's development of a mercury TMDL for the Sacramento and San Joaquin Rivers and associated Delta Estuary (Delta Mercury TMDL), over twenty publicly owned treatment works (POTWs) agreed to collectively share data generated from compliance with the Water Boards order for independent review outside of the Water Board's TMDL process. This report documents the results of that independent review.

Based on the monthly analyses of methylmercury in effluent and, in a limited subset, influent and receiving waters, this study shows that POTWs are an extremely small fraction of the methylmercury load to the Delta. While there is considerable variation among plants with respect to the concentrations of methylmercury measured in effluent discharges, even the plants discharging the highest methylmercury concentrations appear to be small contributors to the overall annual mercury load. The total methylmercury load from all treatment plants in this study is approximately 103 g/yr, with a likely range of 18-353 g/yr. The likely range reflects uncertainties inherent to extrapolating annual averages from this small data set. This is approximately 2-3% of the total annual methylmercury load to the Delta. Since the CVCWA study participants represent approximately 75% of the total effluent flow from all municipal wastewater dischargers in the Central Valley, this is a reasonable comparison of municipal wastewater to watershed loads.

Because the study indicates that minimal methylmercury loads come from POTWs, mercury is identified as a watershed management problem in need of a watershed management solution. Such identification does not negate the many technical and policy challenges associated with methylmercury and its potential impacts on human health and wildlife. Many questions remain, such as: how to effectively implement a fish tissue target for methylmercury; is there a proven correlation between mercury loads and methylmercury; and, how are funding sources going to be developed for watershed projects (e.g., mine restoration, risk communication) that are identified as high priorities?

Document Type: Research Article


Publication date: 2005-01-01

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